The present invention relates to water-jet propulsion systems for watercraft, and more particularly to a protective control system for preventing overspeeding and undesired shutdown of the jet pump prime mover in such propulsion systems due to unloading of the prime mover.
The invention is particularly suitable for use on hydrofoil watercraft in which the hull is supported on struts which have foil systems at their lower ends. When such a craft is driven at a sufficiently high speed, the submerged foils develop lift and support the hull of the craft above the water surface. Such craft can be operated at relatively high speeds as compared to conventional watercraft, and can be designed to be capable of operation in rough water. They are particularly desirable for operation under these conditions since the hull is supported above the surface and a relatively smooth ride is obtained even though the sea may be quite rough.
Hydrofoil craft may, of course, be propelled by any type of propulsion system. Water-jet propulsion systems, however, are very desirable for these craft. In such a system, a water intake is provided in or on one of the struts and takes in water under ram pressure due to the forward movement of the craft through the water. Water entering through the intake is directed to a pump and is accelerated by the pump and discharged rearwardly in a high-velocity jet, resulting in a forwardly directed reaction force which propels the craft. A prime mover of any suitable type is used to drive the pump and provide the desired propulsive force. A relatively simple system is thus provided which is capable of attaining the desired high speed.
As previously mentioned, hydrofoil craft are capable of operating in rough water and this involves a problem in the use of a water-jet propulsion system. When relatively high waves are encountered, it is possible for the water intake to occasionally broach, or break through the water surface. When this occurs, air is drawn into the pump system, sharply reducing the load or causing large fluctuations in the load on the prime mover. This results in essentially unloading the prime mover and causing overspeed so that the usual protective devices operate and shut down the prime mover. This immediately reduces the propulsive force to zero and reduces the speed of the craft sufficiently to cause it to drop to the surface of the water and become hullborne. Several minutes are usually required for the prime mover to be restarted and for the craft to accelerate to high enough speed to again become foilborne. This is undesirable because of discomfort to the passengers due to the sudden change to hullborne conditions, especially in rough water when this condition is most likely to occur, and overspeeding of the pump prime mover due to unloading may involve some possibility of damage to the prime mover.